1. Field of the Invention
This invention concerns a method for testing the degree of wear of banknotes which are at least partially printed with printing inks which are translucent in respect of light in the invisible range of the spectrum.
2. Description of the Prior Art
The determination of the degree of wear of banknotes is known from numerous publications. In these known methods, unprinted portions of a banknote, such as for example the margin of the note (German OS-print 2,310,882) or unprinted portions contained within the surface of a banknote (U.S. Pat. No. 2,950,799), are generally used for this purpose. If the banknote in question has no unprinted portions or if such unprinted portions are not sufficiently large, testing must be carried out within the print format of the note. It must be taken into account that, as compared with the examination of unprinted sections of a note, the light reaching the photodiodes is modulated by the print pattern along the scanning track. In this connection a testing process is mentioned in German OS-print 2,752,412 which proceeds from the concept that an unsoiled banknote has a certain number of contrast jumps of a certain height along a track running through printed sections of a note which corresponds to the existing print pattern. These contrast jumps produce a standard signal when added up as signal values along the track. If a banknote is soiled the contrast is diminished so that the testing level added up in the case of such a note will deviate from the standard level of a clean note to a greater or lesser degree, depending on how soiled the note is. The difference between both of these levels can be taken as a measure for the degree of soiling.
The method produces good results as long as the track used for the formation of the standard signal is exactly followed in subsequent tests. Now if one diverges from the track, and this is more or less inevitable, the photodiode will scan a more or less varied printed pattern in the vicinity of the original track, and this will inevitably lead to changes in the testing level independent of how soiled the banknote in question is. Depending on the amount of divergency, the testing level will therefore be within a scattering range when a multiple of equally soiled banknotes are scanned or when the same note is scanned a number of times, and this excludes the possibility of obtaining a rating of the degree of wear of a note which is within fine tolerances and subdivided into a number of comparison stages.